[Patent document 1] JP 2010-285140 A (US 2010/0120579 A1)
For instance, Patent document 1 proposes such a power transmission unit which uses one of rotors forming a planetary gear mechanism as a start rotor to apply a start rotational force to an internal combustion engine, and further uses another of the rotors as a transmission rotor which is supplied with power of the internal combustion engine. In detail, a one-way bearing is provided between the internal combustion engine and the transmission rotor. Thereby, after start of the internal combustion engine, a rotation speed of the internal combustion engine increases, thereby transmitting power to the transmission rotor via the one-way bearing. As a result, the start of power supply by the internal combustion engine may be controlled simply.
By the way, the inventors have found that in cases where (i) an internal combustion engine is started, (ii) a rotation speed of an internal combustion engine is being controlled so as to follow a rotation speed of a transmission rotor, and (iii) the rotation speed of the transmission rotor changes, a one-way bearing may be engaged suddenly to thereby cause a shock in a power transmission unit. Further, the inventors have found that, in cases where (i) a rotational force of a start rotor is applied to an internal combustion engine, and then (ii) combustion control of the internal combustion engine is started, a rotation speed of the internal combustion engine increases suddenly and a one-way bearing is engaged suddenly, which may cause a shock in a power transmission unit. Yet further, the inventors have found that after the start of the internal combustion engine, a time taken by engagement of the one-way bearing becomes long depending on a drive condition, thus possibly degrading acceleration.